1. Field of the Invention
The present invention relates to high frequency mixers, and more particularly to radio frequency mixers used in communication receivers and other devices.
2. Background Art
Analog multipliers or mixers are typically used in a wide range of communication applications to, for example, up-convert a baseband analog or digital signal for ease of transmission, or down-convert a high frequency signal to baseband for ease of signal processing. For example, in a down-conversion scenario, the mixer can be used in a receiver that receives high frequency signals (such as radio frequency (RF) signals) to provide a signal at a lower intermediate frequency that can then be processed and demodulated by the receiver, e.g. into digital data. The mixer uses a local oscillator (LO) signal that is input to mix with the received high frequency signal to achieve the intermediate frequency signal.
A Gilbert cell mixer is often used in high frequency receivers, since this type of mixer provides reasonable conversion gain (intermediate frequency (IF) power output with respect to the RF power input), good rejection at the RF and LO ports, and a differential IF output connection. A Gilbert cell mixer can be advantageously implemented in CMOS as a straightforward translation of a bipolar Gilbert cell.
FIG. 1 is a schematic diagram of an example of a prior art fully-differential Gilbert cell mixer 10 implemented using CMOS devices. When used as a down converter (from RF to intermediate frequencies), its operation is generally as follows. The voltage from the oscillating RF signal unbalances the current so that the current flip-flops hack and forth between the two devices 12a and 12b. Devices 14a and 14c are “open” when devices 14b and 14d are “closed,” and then this reverses, with devices 14a and 14c “closed” while devices 14b and 14d are “open.” The switching of the devices occurs back and forth at a rate determined by the frequency of the LO signal. These switches direct the AC currents of the LO from devices 12a and 12b to the output load, half the time to one port at the load, and half the time to the other port at the load. This non-linear process produces many frequencies; one of the more prominent frequencies produced is approximately the sum of the RF and LO frequencies, while another prominent frequency is approximately the difference of these frequencies. One of these can be the desired intermediate frequency (IF). Even if devices 14a-14d do not act as switches, the mixer still can produce the desired intermediate frequency, since two of the devices 14a-d will direct more current during on window of time and less current during the alternate window. The load 16 can be an active load or a passive load.
This type of mixer suffers from some undesirable characteristics or disadvantages. Gilbert cells provide a low noise figure, but at the expense of a very low linearity, which figure, can be undesirable in some applications. Another disadvantage is that, when the input transistors 12a and 12b that receive the RF signal are biased at high current, this results in high transconductance and thus high conversion gain. The devices 14a-d that receive the local oscillator (LO) signal exhibit high gate-to-source voltage as a result. This, in turn, disturbs the switching action of those devices 14a-d, which gives rise to distortion and noise.
Another disadvantage is that the noise performance of the Gilbert cell mixer at low frequencies is dominated by the noise that arises due to switching action at the devices 14a-d which receive the LO signal. NMOS switching devices operating at high bias currents, and thus high gate-to-source voltage, exhibit increased output noise. Low noise performance, especially at low frequencies, e.g., at the output of a down-converting mixer, is very critical in receiver architectures, especially in direct conversion architectures, so that the prior art designs may be undesirable in many applications.